1. Field of the Invention
The present invention relates to a reference voltage source for controlling a current source of a coupled-mode logic (CML) circuit. It more particularly relates to the fabrication of a voltage regulator operating at a low, 3-volt, supply voltage.
2. Discussion of the Related Art
There are two main types of coupled-mode logic circuits: emitter-coupled logic (ECL) circuits fabricated from bipolar transistors, and source-coupled logic (SCL) circuits fabricated from MOS transistors.
In any logic circuit, it is desirable to provide the lowest possible supply voltage in order to have a minimum energy consumption. However, the minimum value of the supply voltage is limited by the circuits that are used to provide the current sources of the logic circuit with a steady reference voltage despite variations of the supply voltage Vcc or of the operating temperature, in order to maintain a constant input or output level difference of logic gates. This difference can be expressed for an ECL circuit as being the voltage difference .DELTA.V between two complementary outputs of the logic gate. In an SCL circuit, this difference can be expressed as being the voltage difference .DELTA.V between two complementary inputs of the logic gate.
The difference .DELTA.V between two levels of a logic gate, either in ECL or SCL circuits is, for example, 0.4 volt.
The selection between bipolar or MOS technologies of the logic circuits mainly depends on the use of the circuit. For example, if the circuit requires high switching speeds, logic gates are generally fabricated according to the ECL technique.
In BICMOS circuits using both bipolar and CMOS technologies on the same chip, the two types of logic circuits can be implemented. Then, voltage regulators for controlling the current sources are fabricated with bipolar transistors for the ECL circuits and with MOS transistors for the SCL circuits, respectively. In addition, conventional regulators using bipolar transistors do not make it possible to decrease the supply voltage to approximately 3 volts.